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The Porsche special tool is P228, although the marks were never intended to be used in place of a "real" valve spring installed height micrometer. They would get you in the ball park if you lacked the more accurate tool. If you're doing this whole affair in the car, you'll likely need a borescope tool or mirrors to read the installed height.
When the SC's had the bad batch of valve springs in the 80's, we changed a lot of them in the car and it was hard enough to change the springs, much less adjusting the spring heights. Most of us would just replace the springs and not alter the number of shims. But remember, when under warranty, they weren't 40+ years old and not already been apart and fiddled with a few times.
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